[Update] Use FlashInfer fast_decode_plan directly instead of replication (#34687)

Signed-off-by: Andrii <askliar@nvidia.com>
Co-authored-by: Andrii <askliar@nvidia.com>

tests/kernels/attention/test_flashinfer.py

@@ -84,6 +84,209 @@ def ref_paged_attn(
     return torch.cat(outputs, dim=0)
 
 
+def _make_paged_kv_metadata(
+    kv_lens: list[int],
+    block_size: int,
+    num_blocks: int,
+) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor, torch.Tensor]:
+    """Build paged-KV metadata tensors for fast_plan_decode tests.
+
+    Returns:
+        kv_indptr          – CPU int32, shape [num_seqs + 1]
+        kv_indices         – CUDA int32, shape [total_blocks]
+        kv_last_page_lens  – CPU int32, shape [num_seqs]
+        block_tables       – CUDA int32, shape [num_seqs, max_blocks_per_seq]
+    """
+    num_seqs = len(kv_lens)
+    max_blocks = (max(kv_lens) + block_size - 1) // block_size
+    block_tables = torch.randint(
+        0, num_blocks, (num_seqs, max_blocks), dtype=torch.int32, device="cuda"
+    )
+
+    indptr_list = [0]
+    indices_list: list[int] = []
+    last_lens_list: list[int] = []
+    for i, seq_len in enumerate(kv_lens):
+        n = (seq_len + block_size - 1) // block_size
+        indices_list.extend(block_tables[i, :n].cpu().tolist())
+        indptr_list.append(indptr_list[-1] + n)
+        last_lens_list.append(seq_len % block_size or block_size)
+
+    return (
+        torch.tensor(indptr_list, dtype=torch.int32, device="cpu"),
+        torch.tensor(indices_list, dtype=torch.int32, device="cuda"),
+        torch.tensor(last_lens_list, dtype=torch.int32, device="cpu"),
+        block_tables,
+    )
+
+
+def _make_cg_decode_wrapper(
+    num_seqs: int,
+    kv_indices_buffer: torch.Tensor,
+    workspace_buffer: torch.Tensor,
+    use_tensor_cores: bool = True,
+) -> "flashinfer.BatchDecodeWithPagedKVCacheWrapper":
+    """Create a cudagraph-enabled BatchDecodeWithPagedKVCacheWrapper.
+
+    *kv_indices_buffer* is shared with the caller so that fast_plan_decode
+    can avoid the device-to-device index copy on subsequent (cudagraph) calls.
+    """
+    return flashinfer.BatchDecodeWithPagedKVCacheWrapper(
+        workspace_buffer,
+        "NHD",
+        use_cuda_graph=True,
+        paged_kv_indptr_buffer=torch.zeros(
+            num_seqs + 1, dtype=torch.int32, device="cuda"
+        ),
+        paged_kv_indices_buffer=kv_indices_buffer,
+        paged_kv_last_page_len_buffer=torch.zeros(
+            num_seqs, dtype=torch.int32, device="cuda"
+        ),
+        use_tensor_cores=use_tensor_cores,
+    )
+
+
+def test_fast_decode_plan_importable() -> None:
+    """fast_decode_plan must be importable from flashinfer.decode.
+
+    This is a forward-compatibility smoke test: if FlashInfer reorganises its
+    public API the import will fail before any other test does.
+    """
+    from flashinfer.decode import fast_decode_plan  # noqa: F401
+
+    assert callable(fast_decode_plan)
+
+
+@pytest.mark.parametrize("dtype", DTYPES)
+@torch.inference_mode
+def test_fast_plan_decode_warmup_uses_full_plan(dtype: torch.dtype) -> None:
+    """On the first call fast_plan_decode must route through self.plan() and
+    flip vllm_first_call to False on the wrapper object."""
+    from unittest.mock import patch
+
+    from vllm.v1.attention.backends.flashinfer import fast_plan_decode
+
+    torch.set_default_device("cuda")
+    set_random_seed(0)
+
+    kv_lens = [128, 64]
+    block_size = 16
+    num_seqs = len(kv_lens)
+    num_query_heads, num_kv_heads = 8, 2
+    head_size = 128
+
+    kv_indptr, kv_indices, kv_last_page_lens, _ = _make_paged_kv_metadata(
+        kv_lens, block_size, NUM_BLOCKS
+    )
+
+    workspace = torch.empty(128 * 1024 * 1024, dtype=torch.int8)
+    wrapper = _make_cg_decode_wrapper(num_seqs, kv_indices.clone(), workspace)
+
+    assert getattr(wrapper, "vllm_first_call", True) is True
+
+    with patch.object(wrapper, "plan", wraps=wrapper.plan) as mock_plan:
+        fast_plan_decode(
+            wrapper,
+            indptr_cpu=kv_indptr,
+            indices=kv_indices,
+            last_page_len_cpu=kv_last_page_lens,
+            num_qo_heads=num_query_heads,
+            num_kv_heads=num_kv_heads,
+            head_dim=head_size,
+            page_size=block_size,
+            q_data_type=dtype,
+            kv_data_type=dtype,
+        )
+        mock_plan.assert_called_once()
+
+    assert wrapper.vllm_first_call is False, (
+        "vllm_first_call should be False after the first fast_plan_decode call"
+    )
+
+
+@pytest.mark.parametrize("kv_lens", [[1328, 18, 463], [1, 54, 293, 70]])
+@pytest.mark.parametrize("num_heads", NUM_HEADS)
+@pytest.mark.parametrize("head_size", HEAD_SIZES)
+@pytest.mark.parametrize("block_size", BLOCK_SIZES)
+@pytest.mark.parametrize("dtype", DTYPES)
+@torch.inference_mode
+def test_fast_plan_decode_matches_full_plan(
+    kv_lens: list[int],
+    num_heads: tuple[int, int],
+    head_size: int,
+    block_size: int,
+    dtype: torch.dtype,
+) -> None:
+    """fast_plan_decode's cudagraph path (delegating to FlashInfer's
+    fast_decode_plan) must produce attention output numerically identical to
+    a standard plan() call.
+
+    Both the warmup call (self.plan) and the subsequent fast call
+    (fast_decode_plan) are verified against the same reference.
+    """
+    from vllm.v1.attention.backends.flashinfer import fast_plan_decode
+
+    torch.set_default_device("cuda")
+    set_random_seed(0)
+    num_seqs = len(kv_lens)
+    num_query_heads, num_kv_heads = num_heads
+
+    query = torch.randn(num_seqs, num_query_heads, head_size, dtype=dtype)
+    key_value_cache = torch.randn(
+        NUM_BLOCKS, 2, block_size, num_kv_heads, head_size, dtype=dtype
+    )
+
+    kv_indptr, kv_indices, kv_last_page_lens, _ = _make_paged_kv_metadata(
+        kv_lens, block_size, NUM_BLOCKS
+    )
+
+    # Reference output via the standard plan()
+    workspace_ref = torch.empty(128 * 1024 * 1024, dtype=torch.int8)
+    ref_wrapper = flashinfer.BatchDecodeWithPagedKVCacheWrapper(
+        workspace_ref, "NHD", use_tensor_cores=True
+    )
+    ref_wrapper.plan(
+        kv_indptr,
+        kv_indices,
+        kv_last_page_lens,
+        num_query_heads,
+        num_kv_heads,
+        head_size,
+        block_size,
+        "NONE",
+        q_data_type=dtype,
+        kv_data_type=dtype,
+    )
+    ref_output = ref_wrapper.run(query, key_value_cache)
+
+    # CUDAGraph wrapper exercised through fast_plan_decode
+    kv_indices_buf = kv_indices.clone()
+    workspace_cg = torch.empty(128 * 1024 * 1024, dtype=torch.int8)
+    cg_wrapper = _make_cg_decode_wrapper(num_seqs, kv_indices_buf, workspace_cg)
+
+    plan_kwargs: dict = dict(
+        indptr_cpu=kv_indptr,
+        indices=kv_indices_buf,
+        last_page_len_cpu=kv_last_page_lens,
+        num_qo_heads=num_query_heads,
+        num_kv_heads=num_kv_heads,
+        head_dim=head_size,
+        page_size=block_size,
+        q_data_type=dtype,
+        kv_data_type=dtype,
+    )
+
+    # First call – warmup path (routes through self.plan)
+    fast_plan_decode(cg_wrapper, **plan_kwargs)
+    warmup_output = cg_wrapper.run(query, key_value_cache)
+    torch.testing.assert_close(warmup_output, ref_output, atol=1e-2, rtol=1e-2)
+
+    # Second call – fast path (routes through fast_decode_plan from FlashInfer)
+    fast_plan_decode(cg_wrapper, **plan_kwargs)
+    fast_output = cg_wrapper.run(query, key_value_cache)
+    torch.testing.assert_close(fast_output, ref_output, atol=1e-2, rtol=1e-2)
+
+
 @pytest.mark.parametrize("kv_lens", [[1328, 18, 463], [1, 54, 293, 70]])
 @pytest.mark.parametrize("num_heads", NUM_HEADS)
 @pytest.mark.parametrize("head_size", HEAD_SIZES)